Semiconductor image sensors are used for sensing light. Complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) and charge-coupled device (CCD) sensors are widely used in various applications such as digital still camera or mobile phone camera applications. These devices utilize an array of pixels in a substrate, including photodiodes and transistors, that can absorb radiation projected toward the substrate and convert the sensed radiation into electrical signals.
Semiconductor image sensors use micro-lenses to focus incoming light. A micro-lens can be formed in an embedded manner inside a layer of the image sensor. In that case, an external micro-lens may also be used to complement the embedded micro-lens. Having both an external lens and an embedded lens may be desirable because they improve light sensitivity, for example sensitivity to black and white light. However, traditional methods of forming the embedded micro-lens are complex and time consuming. The embedded micro-lenses formed by the traditional methods also tend to have poor light-focusing performance.
Therefore, while existing methods of fabricating embedded micro-lenses for image sensors have been generally adequate for their intended purposes, they have not been entirely satisfactory in every aspect.